The first generation of computers lasted between 1940 and 1955. Early computers stored information in magnetic drums and relied on power from vacuum tubes. These computers were the size of averaged sized rooms, very large compared to today's computers.
Vacuum Tubes
2nd Generation of Computers
The second generation of computers lasted from 1956 to 1964. They ditched the vacuum tubes for transistors who worked much better. These computers were a good bit smaller, between the size of a half a room to a work desk.
3rd Generation of Computers
The third generation of computers made even more drastic changes, lasting from 1965 to 1971. This generation replaced transistors with integrated circuts and transitioned in to the standard desktop layout that is still used today. These computers where much more ergonomic, the size of slightly larger desktop computers.
4th Generation of Computers
The Fourth generation of computers started in 1971 and is still active today. These computers used microprossesors, which incorporated all the functions of a central processing united to one circuit. These computers continue to get smaller and more ergonomic as new designs come out.
I believe the 5th generation of computers will come in 2040. These computers will form unlimited resolution holographic displays projected by a small, thin keyboard. They will be small enough to fit into a folder.
5th Generation of Computers
I believe the 6th generation of computers will come in 2070. These computers will be the size of a 1cm by 1cm chip but project all of its display onto glasses. Even more powerful than industrial computers and can fit into a pocket.